The existence of bacteria that bind specifically to the plasmaproteinase inhibitor .alpha..sub.2 M has earlier been reported (Muller and Blobel 1983; 1985). The binding of .alpha..sub.2 M to streptococci is highly specific and streptococci of different serological groups or species bind the two different conformational forms of this plasma proteinase inhibitor referred to as "slow" (s.alpha..sub.2 M) and "fast" (f.alpha..sub.2 M) based on their electrophoretical behaviour (Muller and Blobel 1983; 1985). The slow form represents the native inhibitor and the fast form the inhibitor/protease complex (Barret et al. 1979). In 1989 Sjobring et al. reported that both conformational forms of .alpha..sub.2 M could be bound to the protein G. This receptor is one of the best studied so called type III Fc receptors (Bjorck and .ANG.kerstrom 1990, Sjobring et al. 1989a, Sjobring el al. 1991). The gene encoding protein G has been cloned and sequenced (Guss et al. 1986; Olsson et al. 1987) and binding properties been studied (Guss et al. 1986; Bjorck et al. 1987; .ANG.kerstrom et al. 1987; Nygren et al. 1988). As a result of these studies it has been shown that protein G binds both IgG and serum albumins through specific domains in the protein. Sjobring et al. 1989b reported that in addition of binding these two serum proteins protein G should also bind to both conformational forms of .alpha..sub.2 M. The latter binding they reported should be located in the IgG binding domain of protein G,--a finding that will be contradicted in the present application. The quantification of free .alpha..sub.2 M and .alpha..sub.2 M-protease complexes using streptococcal .alpha..sub.2 M receptors have been described (Justus et al. 1990). Also in studies concerning purification and characterisation of .alpha..sub.2 M from mastitis milk these streptococcal receptors have been used (Rantamaki and Muller 1992). In these cases intact streptococcal cells have been used as the source of the receptor. Therefore a protein which for instance binds specifically to .alpha..sub.2 M should be of great biotechnological interest. This protein could in analogy to what Nygren et al. 1988 reported be immobilized and used to affinity purify away any undesired .alpha..sub.2 M in the sample. Furthermore different proteins (or fragments thereof) with .alpha..sub.2 M-binding activity could be used to detect and measure the amount of .alpha..sub.2 M in a sample. Generally it may be difficult to obtain a homogeneous and reproducible product if such receptors should be prepared from streptococcal cells directly. Moreover, most streptococci are pathogenic and need complex culture media which involve complications in large-scale cultures. There is thus a need for a new method for producing .alpha..sub.2 M-binding proteins (or fragments thereof).